This invention relates to an apparatus for precisely moving a table or stage, the apparatus being used in a step and repeat camera, a step and repeat projection printing apparatus, etc.
An apparatus, such as a step and repeat camera, of the type mentioned above, operates to project and print an integrated circuit pattern photographically on a mask substrate or wafer which is usually coated with a photoresist layer. Because of the inherent restriction of the image area of a projection lens, however, it is impossible to print the whole area of the aforecited mask substrate or wafer in a single step. Therefore, a precision X-Y movable table is provided in such equipment and the mask substrate or wafer is installed thereon. While performing intermittent feed operations in the X- and Y-directions, the integrated circuit pattern is arrayed and printed on the entire area of the mask substrate or wafer.
In the production of an integrated circuit, it is required that circuit patterns differing from one another be overlapped on a single wafer with a positioning precision of approximately 0.2 .mu.m, ordinarily 8 to 12 times or so. Therefore, an equal or higher positioning precision is usually required for the positioning precision of the X-Y movable table. Further, it is a requirement that such X-Y movable table be capable of high-speed positioning in order to enhance the productivity of the system.
Therefore, many of the X-Y movable tables of this sort have heretofore been of such construction that a Y-directional movable table is stacked on an X-directional movable table and that a driving system for the Y-directional movement including a motor, feed screws, etc., is installed on the X-directional movable table. Although such a construction attains a comparatively excellent machine precision, the masses of the movable parts inevitably become great and the necessary high-speed positioning as mentioned above is impossible to attain in practice.
As an improvement on such prior art apparatus, there has been proposed an X-Y movable table as shown in FIG. 1 wherein both X- and Y-driving systems are fixed to a support frame, as disclosed in "Kikai Sekkei (Machine Design)," Volume 14, No. 2, 1970, page 35). With this structure, however, in the case where the movable table moves in the Y-axial direction, the guide surfaces for the Y-directional movable table are formed by surfaces of the X-frame portion 1 and the Y-frame portion 2, with the result that the straightness of the moving direction of the table cannot be maintained accurately. In addition, there are many rolling guide surfaces associated with these frame members, which surfaces are required to be hard to prevent rapid wear, and long periods of time are required for working and for assemblage and adjustment of this apparatus in order to ensure that the precision of the movable table is higher.
In an apparatus of this sort, these problems can be somewhat solved by replacing the rolling guide with a sliding guide which employs a higher polymer material for the sliders. More specifically, since a sliding guide does not require the same degree of hardness of the guide surfaces to prevent wear as does the rolling guide, the working of the members is facilitated, and since the contact areas between the sliders and the guide surfaces can be made large, minute form errors in the guide surfaces are averaged, so that the sliding guide is capable of movements of higher precision as compared with the rolling guide. Further, the sliding friction exhibited by the sliding guide provides a more rapid settling of the table after its movement so that the positioning can be made much faster than that in the case of the rolling guide.
However, even when such a sliding guide is utilized, the X-Y two dimensional movable table as described above incurs another disadvantage in contrast to the aforecited advantages. That is, strain energy which corresponds to the parallelism error between both of the guide surfaces of the X-frame portion 1 and the Y-frame portion 2 in FIG. 1 is accummulated in the frame members, etc., and this brings about the disadvantage that yaw or stick slip is caused. This is attributed to the frictional forces of the sliding parts. A further disadvantage is that the strain energy is gradually released to give rise to a phenomenon in which the movable table drifts at a very low speed of, for example, 0.1 to 0.5 .mu.m/sec after the positioning thereof.